What keeps a republic to its intended purpose, governed by the rule of law that is based on individual rights, vs. changes to that law that would transform it into a de facto democracy and dictatorship?

The answer is education. An educated populace will not fall for a scam in sufficient numbers to provide the "super majority" needed to amend and corrupt its Constitution. That limits any would-be changes to it and subdues any knee-jerk reactions to daily events, disarming those who would take advantage of such occurrences for their own gain.

This is the importance of a "liberal education" that includes history and politics: a general awareness of what has worked, or not, and why. A republic survives on the awareness of its citizens. It must; its rule is given on their authority, and they must be able and willing to maintain that resposibility, unflinchingly and explicitly.

Their awareness is the foundation of the on-going life of a republic and a bulwark against its enemies. Those enemies know that if they can destroy a country's education, they can destroy its ability to resist the diseases of subversion and decay. But rebuilding, restoring, and maintaining education to its proper role and purpose restores the citizens' presence and awareness, and thereby maintains the health and strength of their republic.